Ion transport across cellular membranes is essential to many of life's processes, such as electrical signaling in nerves, muscles, and synapses or cell's maintenance of homeostatic balance. Biological systems achieve rapid, selective and ultra-efficient trans-membrane mass transport by employing a large variety of specialized protein channels of nanometer or subnanometer size (Hille B (2001) Ion Channel of Excitable Membranes (Sinauer Associates, Inc., Sunderland)). High resolution x-ray structures, protein sequencing, targeted mutations, and biophysical characterizations have provided new insights on the link between nanochannel protein architecture, transport rates, selectivity, and gating properties.
Interestingly, these studies have shown that membrane nanochannels share several common features. For example, aquaporins (Sui H X, Han B G, Lee J K, Walian P, Jap B K (2001) Structural basis of water-specific transport through the AQP1 water channel. Nature 414:872-878; and Murata K, Mitsuoka K, Hirai T, Walz T, Agre P, Heymann J B, Engel A, Fujiyoshi Y (2000) Structural determinants of water permeation through aquaporin-1. Nature 407:599-605), proton (Wikstrom M (1998) Proton translocation by bacteriorhodopsin and heme-copper oxidases. Curr Optin Struct Biol 8:480-488, Wikstrom M, Verkhovsky M I, Hummer G (2003) Water-gated mechanism of proton translocation by cytochrome c oxidase. BBA-Bioenergetics 1604:61-65), and ion channels (Jiang Y X, Lee A, Chen J Y, Cadene M, Chait B T, MacKinnon R (2002) Crystal structure and mechanism of a calcium-gated potassium channel. Nature 417:515-522; Jiang Y X, Lee A, Chen J Y, Cadene M, Chair B T, MacKinnon R (2002) The open pore conformation of potassium channels. Nature 417:523-526; Bass R B, Strop P, Barclay M, Rees D C (2002) Crystal structure of Escherichia coli MscS, a voltage-modulated and mechanosensitive channel. Science 298:1582-1587; Doyle D A, Cabral J M, Pfuetzner R A, Kuo A L, Gulbis J M, Cohen S L, Chait B T, MacKinnon R (1998) The structure of the potassium channel: Molecular basis of K+ conduction and selectivity. Science 280:69-77; Miyazawa A, Fujiyoshi Y, Unwin N (2003) Structure and gating mechanism of the acetylcholine receptor pore. Nature 423:949-955; and Kuo A L, Gulbis J M, Antcliff J F, Rahman T, Love E D, Zimmer J, Cuthbertson J, Ashcroft F M, Ezaki T, Doyle D A (2003) Crystal structure of the potassium channel KirBacl.1 in the closed state. Science 300:1922-1926) all have relatively narrow and hydrophobic pore regions. By contrast, the selectivity filter regions of membrane ion channels are enriched with charged residues.
Despite progress made in recent decades, the complex macromolecular nature of these biological machines still complicates the understanding of the underlying mechanisms responsible for fast mass transport, selectivity, gating, and the functional role of hydrophobic pore lining and charged functionalities. Thus, it is desirable to create simplified, biomimetic nanochannels that could help to clarify the physics of ion permeation at the nanoscale, as well as create the next generation of membranes that employ efficient molecular transport for applications ranging from water purification to separations of biomolecules. Recent theoretical and experimental works have proposed carbon nanotubes (CNTs) as candidates for such simplified models of biological channels. The graphite walls of CNTs form hydrophobic pores with diameters close to those of biological channels.
Molecular dynamics (MD) and theoretical studies have shown single-file transport for water along the nanotube axis (Berezhkovskii A, Hummer G (2002) Single-file transport of water molecules through a carbon nanotube. Phys Rev Lett 89:4; Hummer G, Rasaiah J C, Noworyta J P (2001) Water conduction through the hydrophobic channel of a carbon nanotube. Nature 414:188-190; Hummer G (2007) Water, proton, and ion transport: from nanotubes to proteins. Mol Phys 105:201-207; and Kalra A, Garde S, Hummer G (2003) Osmotic water transport through carbon nanotube membranes. Proc Natl Acad Sci USA 100:10175-10180) that is reminiscent of the water wires observed in aquaporins (Sui H X, Han B G, Lee J K, Walian P, Jap B K (2001) Structural basis of water-specific transport through the AQP1 water channel. Nature 414:872-878; and Murata K, Mitsuoka K, Hirai T, Walz T, Agre P, Heymann J B, Engel A, Fujiyoshi Y (2000) Structural determinants of water permeation through aquaporin-1. Nature 407:599-605). Predicted (Hummer G, Rasaiah J C, Noworyta J P (2001) Water conduction through the hydrophobic channel of a carbon nanotube. Nature 414:188-190; Hummer G (2007) Water, proton, and ion transport: from nanotubes to proteins. Mol Phys 105:201-207; and Kalra A, Garde S, Hummer G (2003) Osmotic water transport through carbon nanotube membranes. Proc Natl Acad Sci USA 100:10175-10180) and experimentally measured (Holt J K, Park H G, Wang Y M, Stadermann M, Artyukhin A B, Grigotopoulos C P, Noy A, Bakajin O (2006) Fast mass transport through sub-2-nanometer carbon nanotubes. Science 312:1034-1037; and Majumder M, Chopra N, Andrews R, Hinds B J (2005) Nanoscale hydrodynamics—Enhanced flow in carbon nanotubes. Nature 438:44-44) water transport rates through CNTs are extremely large and comparable to measured values for aquaporins. MD simulations have revealed the water ordering near the smooth hydrophobic walls to facilitate enhanced, frictionless water transport.
In addition, chemical inertness of the carbon nanotube sidewalls facilitate specific functionalization of the CNT pore entrance with different functionalities. This specificity provides an opportunity to create an artificial “selectivity filter” that could impart gating properties to a CNT (Hinds B J, Chopra N, Rantell T, Andrews R, Gavalas V, Bachas L G (2004) Aligned multiwalled carbon nanotube membranes. Science 303:62-65; Majumder M, Chopra N, Hinds B J (2005) Effect of tip functionalization on transport through vertically oriented carbon nanotube membranes. J Am Chem Soc 127:9062-9070; Majumder M, Zhan X, Andrews R, Hinds B J (2007) Voltage gated carbon nanotube membranes. Langmuir 23:8624-8631; Nednoor P, Chopra N, Gavalas V, Bachas L G, Hinds B J (2005) Reversible biochemical switching of ionic transport through aligned carbon nanotube membranes. Chem Mater 17:3595-3599; Nednoor P, Gavalas V G, Chopra N, Hinds B J, Bachas L G (2007) Carbon nanotube based biomimetic membranes: Mimicking protein channels regulated by phosphorylation. J Mater Chem 17:1755-1757; and Chopra N, Majumder M, Hinds B J (2005) Bifunctional carbon nanotubes by sidewall protection. Adv Funct Mater 15:858-864).
Reverse osmosis (RO) is currently a method for desalination of sea water. Sea water is an abundant reservoir of elemental water on Earth and as such is one of the important potential sources of fresh water that may be necessary for normal society functioning. Seawater has high salinity, which makes it unsuitable for most of human use, therefore seawater needs to be separated from its salt content in the desalination process.
In a typical RO desalination process, an applied pressure in excess of the osmotic pressure of the salt solution forces the solution through a semipermeable membrane that allows permeation of water while retaining the dissolved ions. This process requires high pressure on the high concentration side of the membrane, ranging from ˜15 bar for brackish water to ˜60 bar for seawater. Fresh water then collects on the downstream side of the membrane and the concentrated brine from the upstream side of the membrane is then discarded.
Current membranes used for RO desalination are based on cellulose acetate or aromatic polyamide polymers and present a thin dense barrier layer in the polymer matrix where most separation occurs. Since the barrier layer is effectively non-porous, the transport of water through the membrane occurs at low rates through a “solution-diffusion” mechanism: water absorbs on the upstream side of the membrane, diffuses down the chemical potential gradient (largely due to pressure gradient), and desorbs downstream. Salt transport occurs in a similar fashion; however, the driving force for transport is mainly the concentration gradient and the salt flux is insensitive to the pressure gradient. Thus, to achieve good water fluxes and high salt rejection, a very large applied pressure is required. As a consequence, the energy cost associated to the separation process is large. Also, these membranes may tend to foul easily.
Biological pores regulate the cellular traffic of a large variety of solutes, often with high selectivity and fast flow rates. These pores share several common structural features: the inner surface of the pore is frequently lined with hydrophobic residues and the selectivity filter regions often contain charged functional groups. Hydrophobic, narrow diameter carbon nanotubes can provide a simplified model of membrane channels by reproducing these critical features in a simpler and more robust platform. Previous studies demonstrated that carbon nanotube pores can support a water flux comparable to natural aquaporin channels.